How can I set sliders to values outside the limits allowed by their slider?
The values you can enter in with the mouse are clamped to (soft limits), 
if type your value in specifically with the keyboard you can get an increased range (hard limits):



Note that once you type in a value higher than the initial soft limit, you will be able to slide the value to the next highest soft limit. 

e.g. for subdivide, if you type in 11 (the initial soft limit is 10), you can slide the value to 20, if you type 21, you can slide it to 30, etc.

This applies to many such limits, not just subsurf.